The present invention relates to a process of fabricating a thin film semiconductor device in which thin film transistors, each including as an active layer a semiconducting thin film formed on an insulating substrate, are integratedly formed. In particular, the present invention concerns a laser beam irradiation technique (laser annealing) for crystallization of a semiconducting thin film having formed on an insulating substrate.
A laser annealing treatment using a laser beam has been developed as one of processes of fabricating a thin film semiconductor device at low temperatures. The laser annealing treatment includes steps of locally heating and melting a semiconducting thin film formed from a non-single crystal material such as amorphous silicon or polycrystalline silicon on an insulating substrate by irradiating a laser beam to the semiconducting thin film, and then crystallizing the semiconducting thin film in the cooling step. Thin film transistors, each including the semiconducting thin film thus crystallized as an active layer (channel region), are integratedly formed. Since the carrier mobility of the semiconducting thin film becomes higher by crystallization, characteristics of the thin film transistor including such a film can be improved. As shown in FIG. 10, in the laser annealing, a pulsed laser beam 4 formed in a band-shape along the longitudinal direction (Y direction) of an insulating substrate 1 is intermittently irradiated to the insulating substrate 1, and it is simultaneously moved relative to the insulating substrate 1 in the lateral direction (X direction) while partially overlapping regions irradiated with the laser beam 4 to each other. In the example shown in FIG. 10, the insulating substrate 10 is steppedly moved in the -X direction while the area irradiated with the laser beam 4 is fixed. The crystallization of a semiconducting thin film can be thus relatively uniformly performed by overlappingly irradiating the laser beam 4 to the semiconducting thin film.
Thin film semiconductor devices are suitable for drive substrates of active matrix type displays or the like, and in recent years they are being increasingly developed. In the case of using a thin film semiconductor device for a display, it is strongly required to enlarge the size of a transparent insulating substrate made of, for example, glass or the like and to reduce the cost thereof. In the example shown in FIG. 10, the insulating substrate 1 has dimensions of 400-500 mm in the X-direction and 300-400 mm in the Y-direction. To satisfy such requirements for enlargement in size and reduction in cost of a transparent insulating substrate, the laser annealing treatment using a laser beam has been adopted. That is, a semiconducting thin film can be crystallized at a relatively low temperature by irradiation of a laser beam thereto, and consequently a relatively inexpensive transparent insulating substrate made of low melting point glass or the like can be adopted. Thus, at present, a thin film semiconductor device integrally containing a peripheral circuit unit in addition to a display unit can be fabricated at low temperatures of 400xc2x0 C. or less using bottom gate type thin film transistors. Further, a semiconducting thin film having a relatively large area can be efficiently converted from an amorphous phase into a polycrystalline phase by irradiating a band-shaped (linear) laser beam 4 to the semiconducting thin film while overlappingly scanning the laser beam 4. At the present time, an excimer laser is extensively used as a laser beam light source. The excimer laser, however, cannot make extremely larger the cross-section of the laser beam by the limited output power thereof. For this reason, a laser beam formed in a band-shape or linear shape is overlappingly scanned to be thus irradiated on the entire surface of a large-sized transparent insulating substrate made of glass or the like. In this case, however, upon scanning of the laser beam, particle sizes of crystals or the like of the semiconducting thin film become uneven by the influence of an energy distribution of the laser beam 4. This causes a problem in varying operational characteristics of drive thin film transistors integratedly formed in a display, thereby making it difficult to perform uniform display.
In general, an excimer laser has an output power of about 200 W. As shown in FIG. 11, a laser beam 4 is formed in a band-shape for concentration of the power. In the example shown in FIG. 11, an area irradiated with the band-shaped laser beam 4 has dimensions of about 0.3 mm (300 xcexcm) in the X direction and about 150 mm in the Y direction. Such a laser beam 4 is intermittently irradiated on the insulating substrate while being scanned along the x direction, to thereby recrystallize a semiconducting thin film formed on the entire surface of the insulating substrate.
FIG. 12 typically shows an energy distribution of the band-shaped laser beam 4 in the X direction (lateral direction). The energy distribution has an approximately parallelopiped profile composed of a flat section 410 at a central portion and tilted sections 420 on both sides thereof. The width of the flat section 410 is, for example, about 300 xcexcm and the width of the tilted section is, for example, about 20 xcexcm. The tilted section 420 is necessarily generated by action of an optic system used for forming the laser beam into a band-shape. The tilted sections 420 of the energy distribution of the laser beam 4 cause a variation in structure of a crystallized semiconducting thin film, resulting in crystal defects.
FIG. 13 typically shows a state in which a laser beam is irradiated while being overlappingly scanned. In a related art laser annealing treatment, a pulsed laser beam is intermittently irradiated while it is scanned such that regions irradiated with laser beam are, for example, 90% overlapped. In the case where the width of the laser beam in the X-direction is 300 xcexcm, the movement amount per one step of the intermittent irradiation becomes 30 xcexcm. In the figure, the movement amount per one step is expressed by a movement step A. By repeating 10 times the intermittent irradiation of the laser beam with the movement amount pitch A, the laser beam is scanned 300 xcexcm in width along the X direction. In this case, the tilted section of the cross-sectional profile of the laser beam is irradiated just at each boundary 16 of the partially overlapped regions irradiated with the laser beam, with a result that crystal defects 16a are generated along each boundary 16. On the other hand, thin film transistors 17 are integratedly formed on an insulating substrate 1 with a specific arrangement pitch B. In this example, the thin film transistor 17 is of a bottom gate type in which a semiconducting thin film 2 patterned into an island is overlapped on the gate electrode 18. A portion of the semiconducting thin film positioned directly over the gate electrode 18 constitutes a channel region, and contacts 19 are formed on both sides of the channel region. While FIG. 13 shows the thin film transistor 17 in a finished state, the laser annealing is performed at a suitable step of a process of fabricating the thin film transistor 17. Any relationship between the movement pitch A of the laser beam and the arrangement pitch B of the thin film transistors has been not examined. Consequently, there occurs a phenomenon that the crystal defects 16a formed for each boundary 16 are positioned in the channel region of one thin film transistor 17 but they are not positioned in the channel region of another thin film transistor 17.
FIG. 14 typically shows the cross-sectional structure of the thin film transistor 17 shown in FIG. 13, in which the gate electrode 18 is patterned on the insulating substrate 1 and the semiconducting thin film 2 is patterned on the gate electrode 18 through a gate insulating film 21. A stopper 23 aligned with the gate electrode 18 is formed on the semiconducting thin film 2. A portion directly under the stopper 23 constitutes a channel region 22. The thin film transistor 17 having such a bottom gate structure is covered with an interlayer insulating film 24. The interlayer insulating film 24 has contact holes through which a source electrode S and a drain electrode D are provided. Portions of the thin film transistor 2 brought in contact with the electrodes S, D are formed of, for example, N+-type diffused layer. In the example shown in FIG. 14, the crystal defects 16a are present just in the channel region 22. The formation of the crystal defects 16a in the channel region 22 of the thin film transistor 17 degrades characteristics of the transistor, in particular, significantly reduces the current driving ability of the transistor. Accordingly, in the case where such a thin film transistor 17 is used as a switching element for a pixel, there occurs unevenness of display of the screen.
FIG. 15 is a graph showing a relationship between a gate voltage VGS and a drain current IDS of a thin film transistor. In this graph, a curve indicated by a solid line shows a transistor characteristic of the thin film transistor containing no crystal defect in a channel region; while a curve indicated by a dot line shows a transistor characteristic of a thin film transistor containing crystal defects in a channel region. The graph of FIG. 15 shows that a thin film transistor containing crystal defects in a channel region is reduced in current driving ability and thereby deteriorated in characteristic of writing a video signal into a pixel. If a variation in operational characteristic of such a thin film transistor appears on a screen of an active matrix type display in the horizontal direction, there occurs an image failure such as a vertical streak.
FIG. 16 typically shows a cause of reducing the current driving ability of a thin film transistor. In the case where a semiconducting thin film made of silicon or the like is melted by laser annealing and is then solidified, the cooling of the film starts from an end portion of each region irradiated with the laser beam, and accordingly, dangling bonds of Si leading to crystal defects are formed after crystallization at each boundary of the partially overlapped regions irradiated with the laser beam. The crystal defect is a portion in which a bonding network of Si is disturbed, and contains a number of local levels. As a result, the crystal defect contains a number of traps of electric charges, to obstruct migration of electrons, thereby reducing the carrier mobility of the semiconducting thin film.
Although the overlapped amount of the laser beam formed in a band-shape is set at 90% in the above related art example, there may be considered a method of increasing the overlapped amount up to 95-99% for further improving the crystal state of a semiconducting thin film. Such a method, however, fails to basically improve the crystal state of a semiconducting thin film because if the overlapped amount is increased up to 95-99%, crystal defects possibly remain at each boundary of the irradiated regions. Japanese Patent Laid-open No. Hei 3-273621 discloses a technique that a laser beam is irradiated only to an element region. However, in this technique, it is impossible to perform the overlapping irradiation (multi-irradiation) of a laser beam, so that it is difficult to significantly improve the crystal quality of a semiconducting thin film present in the element region.
An object of the present invention is to provide a thin film semiconductor device characterized by enhancing electric characteristics of thin film transistors of the thin film semiconductor device by an improved laser annealing treatment.
To achieve the above object, according to a first aspect of the present invention, there is provided a process of fabricating a thin film semiconductor device, including the steps of: forming a semiconducting thin film on the surface of an insulating substrate spread in longitudinal and lateral directions; laser-annealing the semiconducting thin film by intermittently irradiating a pulsed laser beam formed in a band-shape along the longitudinal direction of the insulating substrate to the insulating substrate thereby crystallizing the semiconducting thin film; and integratedly forming thin film transistors, each including the semiconducting thin film as an active layer, with a specific arrangement pitch; wherein said laser annealing step further comprises a step of moving the laser beam relative to the insulating substrate in the lateral direction with a specific movement pitch while partially overlapping regions irradiated with the laser beam to each other, the movement pitch of the laser beam being set at a value equal to an arrangement pitch of the thin film transistors or at a value larger by a factor of an integer than the arrangement pitch of the thin film transistors. The laser annealing step is preferably performed such that any one of boundaries of the partially overlapped regions irradiated with the laser beam is not overlapped on a channel region of each of the thin film transistors.
According to a second aspect of the present invention, there is provided a laser annealing apparatus used for the above fabrication process, including means for intermittently irradiating a pulsed leaser beam formed in a band-shape along the longitudinal direction of the insulating substrate to the insulating substrate, and simultaneously moving the laser beam relative to the insulating substrate in the lateral direction with a specific movement pitch while partially overlapping regions irradiated with the laser beam to each other; means for setting the movement pitch of the laser beam at a value equal to an arrangement pitch of the thin film transistors or at a value larger by a factor of an integer than the arrangement pitch of the thin film transistors; and means for previously positioning the insulating substrate such that any one of boundaries of the partially overlapped irradiated regions is not overlapped on a channel region of each of the thin film transistors.
According to a third aspect of the present invention, there is provided a semiconductor device suitable for the above fabrication process, including: an alignment mark used for positioning said insulating substrate such that any one of boundaries of the partially overlapped irradiated regions formed on said insulating substrate is not overlapped on a channel region of each of said thin film transistors. The insulating substrate preferably includes a pixel electrode corresponding to each of said thin film transistors integratedly formed at a specific arrangement pitch. The thin film semiconductor device of the present invention is suitably used as a drive substrate of an active matrix type display.
According to the present invention, a band-shaped or linear-shaped laser beam is intermittently irradiated to an insulating substrate and it is simultaneously scanned while partially overlapping regions irradiated with the laser beam to each other, wherein the movement pitch of the laser beam is set at a value equal to an arrangement pitch of thin film transistors or at a value larger by a factor of an integer than the arrangement pitch of the thin film transistors. Accordingly, crystal states of the semiconducting thin film crystallized by the laser beam irradiation are distributed repeatedly at the same period as the movement pitch, and the repeated period of the crystal states corresponds to the arrangement pitch of the thin film transistors. As a result, portions of the semiconducting thin film constituting element regions of the thin film transistors integratedly formed on the insulating substrate have the crystal states being substantially similar to each other over the entire surface of the insulating substrate, so that there occurs no variation in operational characteristics of the thin film transistors. Further, according to the present invention, the insulating substrate is previously positioned such that any one of boundaries of the partially overlapped regions irradiated with the laser beam is not overlapped on a channel region of each thin film transistor, and consequently crystal defects present at the boundaries of the irradiated regions are not contained in the channel region of each thin film transistor and thereby all of the thin film transistors have adequately high current drive abilities.